Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Chemical analysis
Reexamination Certificate
2001-02-27
2004-03-30
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Chemical analysis
C702S052000, C702S053000, C073S861000, C073S146000, C073S052000, C073S047000, C204S401000, C204S425000
Reexamination Certificate
active
06714876
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control system using a gas sensor element for various control operations, such as controlling the introduction of outside air into the passenger compartment of an automobile by detecting a variation in the concentration of a gas in the environment. More particularly, the invention relates to a control system using a gas sensor element capable of lessening the effect of variations in sensor resistance among gas sensor elements as well as the effect of variation in the sensor resistance of the gas sensor element caused by environmental factors, such as temperature and humidity.
2. Description of the Related Art
Since the sensor resistance of a gas sensor element using a WO
3
thin film, lead-phthalocyanine, or SnO
2
varies with the concentration of a specific gas, such as NOx, CO, or HC (hydrocarbon) contained in the environment, a conventionally known gas sensor element detects a variation in the concentration of a specific gas as a function of sensor resistance. For example, by using such a gas sensor element, a known control system opens/closes a flap for introducing outside air into the passenger compartment of an automobile according to the condition of contamination of outside air, or controls an air cleaner upon detection of contamination of air within the passenger compartment due to smoking.
In many cases, a control system using such a gas sensor element detects variation in sensor resistance in the form of an electric signal in the following manner. A gas sensor element having a sensor resistance Rs, and a detection resistor having a predetermined detection resistance Rd are connected in series. A predetermined direct-current voltage is applied between opposite ends to thereby divide the voltage by means of the gas sensor element and the detection resistor. On the basis of a divided voltage appearing from a point between the gas sensor element and the detection resistor, various processes are performed.
However, the sensor resistance Rs of a gas sensor element may be greatly influenced by factors, such as temperature and humidity, of the environment in which the gas sensor element is placed, as well as by the concentration of a specific gas, such as NOx, to be detected. Due to environmental factors, such as temperature and humidity, the sensor resistance Rs of the gas sensor element and the detection resistance Rd of a detection resistor may differ greatly in an arrangement where the concentration of a specific gas is determined by detecting a variation in the potential obtained by applying a predetermined voltage to a voltage divider comprising the gas sensor element and the detection resistor as described above. As a result, the potential obtained by the voltage divider may be biased near the predetermined potential or ground potential. Thus, the sensor resistance Rs; i.e., variation in the concentration of a specific gas, cannot be detected accurately.
Also, since sensor properties are not completely uniform among gas sensor elements, even when similar gas sensor elements are used, the sensor resistance Rs; i.e., an output, may vary among the gas sensor elements.
SUMMARY OF THE INVENTION
An object of the invention is to provide a control system using a gas sensor element capable of lessening the effect of variations in sensor properties among gas sensor elements as well as the effect of environmental factors, such as temperature and humidity, to thereby accurately detect variation in the concentration of a specific gas.
To achieve the above object, the present invention provides a control system for a gas sensor element whose sensor resistance varies with the concentration of a specific gas, comprising: a pulse input point into which a pulse signal is input in a repetitive waveform having a first potential state and a second potential state; a capacitor; a charging circuit for charging the capacitor via a charge resistor during a period when the pulse signal in the first potential state is input to the pulse input point; and a discharging circuit for discharging the capacitor via a discharge resistor during a period when the pulse signal in the second potential state is input to the pulse input point. The gas sensor element having a sensor resistance comprises at least either the charge resistor of the charging circuit or the discharge resistor of the discharging circuit. Furthermore, at least either the charging current of the charging circuit or discharging current of the discharging circuit varies with the sensor resistance of the gas sensor element. The control system further comprises a control circuit, which in turn comprises a microcomputer; and an A/D converter circuit for converting a potential at an operating point located at one end of the capacitor to a digital valve, which potential varies with the sensor resistance of the gas sensor element. The control circuit is connected to the pulse input point and outputs the pulse signal.
In the control system using a gas sensor element of the above embodiment, the capacitor is charged and discharged by means of the pulse signal. Also, at least either the charging current during charging or discharging current during discharge varies with the sensor resistance of the gas sensor element. The voltage across the capacitor becomes steady by repeated charging and discharging according to the pulse signal, and the charging voltage of the capacitor (a voltage as measured across the capacitor) varies with the sensor resistance. Thus, when the sensor resistance of the gas sensor element varies as a result of the gas sensor element detecting the specific gas, the charging voltage of the capacitor varies accordingly. As a result, the potential of the operating point located at one end of the capacitor varies accordingly. Thus, an A/D-converted value of the potential varies according to the concentration of the specific gas. Therefore, variation in the concentration of the specific gas can be known from the A/D-converted value.
In the control system, the charging voltage of the capacitor can be varied by varying the duty ratio or the amplitude (the difference between the first potential and the second potential) of the pulse signal, which is output from the control circuit and is input to the pulse input point. Accordingly, when the sensor resistance of the gas sensor element varies due to variation in environmental factors, such as temperature or humidity, the duty ratio, for example, of the pulse signal is varied appropriately to prevent large biasing of the potential at the operating point or an A/D-converted value of the potential, thereby maintaining the value within an appropriate range. Thus, even in this case, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be reliably determined. Similarly, even when sensor properties, such as sensor resistance (for example, a sensor resistance as obtained in an environment of a standard gas concentration at predetermined temperature and humidity), vary among gas sensor elements, the duty ratio, for example, of the pulse signal is varied appropriately in order to prevent large biasing of the potential at the operating point (an A/D-converted value of the potential), thereby maintaining the value within an appropriate range. Thus, variation in the potential at the operating point associated with variation in the concentration of the specific gas can be determined.
The pulse signal is not particularly limited, so long as the pulse signal has a waveform which alternates between a first potential state and a second potential state. The first potential and the second potential may be set for use with a single-polarity power source; for example, either the first potential or the second potential assumes +5V, whereas the other assumes 0 V (ground). Alternatively, the first potential and the second potential may be set for use with a dual-polarity power source; for example, either the first potential or the second potential
Kimoto Yuji
Matsuoka Toshiya
Barlow John
Bhat Aditya S
NGK Spark Plug Co. Ltd.
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